It is known to provide a solar hot water heating system employing a gas-liquid, phase change medium such as the refrigerant commonly used in refrigerators, Freon being one of them. If the water heating tank or condensor is located physically above the outlet of the solar collector the refrigerant will circulate without need for a pump. It rises from the heat collector in its vapor phase, passes through the condensor where it is allowed to condense and then falls in liquid phase by gravity back to the collector inlet. In such a system the volume of hot water available is limited by the size of the heating tank and the collector must be turned off when all the water is hot. Furthermore, the cold water fed into the heating tank will be untempered and hence, when there is demand for hot water from the heating tank, fresh water at its coldest temperature immediately enters the heating tank and cools the water in it.
It is thus apparent that on a bright, sunny day the collector may have considerably excess capacity which cannot be used but that, when the system is activated, it must heat the cold water from the temperature of the water main or well, thus placing a maximum load on the system whenever operating.
It is also known to provide a heat exchanger of negligible hot water storage capacity at a location above the solar collector and an insulated hot water storage tank at a location below the collector, presumably the basement of a house. See U.S. Pat. No. 4,120,289. Such an arrangement requires that all the water to be heated be pumped continuously when the system is operating so as to build up a reservoir of heated water in the remote insulated tank, with attendant waste of energy and heat loss.